The invention relates to instruments for spectral analysis, and particularly to instruments have a tunable etalon as an optical element.
In spectral analysis it is sometimes necessary to align one of the passbands of an etalon with the quasi monochromatic output of another element. For example, in the above identified Wolga patent a tunable acoustic optical filter is used in series with an etalon. It is necessary to align one passband of the etalon with the passband of the acoustic-optic filter.
The cited Wolga device achieve frequency alignment and lock by dithering one of the optical elements. The misalignment of the combination is sensed by sending white light through the filter and carrying out phase-sensitive detection of the variation in total transmitted light intensity as one element is dithered.
The object of the present invention is to provide an improved instrument operating on a DC principle which eliminates the need for dithering. One benefical result is simplication of the electronics required to support the instrument.